Electron gun for cathode-ray tubes



March 16, 1954 R. A. 1.. COLE ET AL 2,672,568

ELECTRON GUN FOR CATHODE-RAY TUBES Filed March 27, 1951 2 Sheets-Sheet 1 Inventors AEG/NALD A. L. COLE ANTHONY V KAAUSE A ttorn e March 16, 1954 COLE 1- AL 2,672,568

ELECTRON GUN FOR CATHODE-RAY TUBES Filed March 27, 1951 2 Sheets-Sheet 2 Inventors REGINALD/l. L. COLE A Home y Patented Mar. 16, 1954 2,672,568 ELECTRON GUN FOR CATHODE-RAY TUBES Reginald Alexander Lister Cole and Anthony Victor Krause, London, England, assignors to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application March 27, 1951, Serial No. 217,746

3 Claims.

The present invention relates to cathode ray tubes and is particularly concerned with the construction of electron guns for tubes having a viewing screen which is scanned by an electron beam of variable intensity.

In a cathode ray tube to be used for viewing purposes, such as in a television receiver, a beam of electrons is arranged to be focused on a fluorescent screen and to be deflected over the surface of the said screen in some form of a raster, usually a series of horizontal lines. The picture intelligence is obtained by varying the electron current and hence the light intensity from point to point over the screen. The present specification is concerned with the production and the current density modulation of an electron beam which illuminates an aperture, the image of which is then focused on the screen by any of the known arrangements with which we are not primarily concerned. In accordance with normal practice, we define the system of electrodes necessary for producing, modulating and focussing the electron beam upon the said aperture as an electron gun and, in what follows, we shall assume that the said aperture is formed in one of the electrodes of the gun to constitute the final aperture thereof.

The essential requirements of an electron gun in a tube of the above mentioned type are that it shall be capable of projecting onto the viewing screen an electron beam to form a spot which is adequately small in size and whoselight intensity varies linearly over a wid range of values in response to a narrow range of variation of the voltage applied to a modulating electrode of the gun, while the ratio of useful beam current to total cathode current should remain as large as possible. In addition, the mechanical construction of the gun and the circuit arrangements for operating it should be as simple as possible.

Since, for a final apertur of given size in-the electron gun, assuming that the beam always completely fills the aperture, the size of pot is, to a first approximation, independent of the type of gun used, we are not directly concerned with the first of the above-mentioned requirements, but the other requirements are of fundamental importance in the design of an electron gun.

In the conventional electron gun as heretofore used in television tubes, electrode systems have been used which, from the point of view of electron current density control, are equivalent to thermionic amplifier valves of the triode or multigrid types. The requirement relating to divergence of the electron beam, and this is adversely affected by close spacing.

It is common practice to bring the electron beam to a first focus or crossover between the cathode and the final gun aperture and then to refocus the beam to form a further crossover inthe final aperture. In the present specification the term crossover refers as is usual in the art, not only to the intersection of electron paths with the beam axis, but includes a constriction of the beam at a position of minimum crosssection.

The most serious difliculties to be overcome in achieving the requirements of the electron gun are due to lens aberrations and space charge effects.

Lens aberrations in the region of the cathode have been severe in many of the guns heretofore proposed; for modulation usually results not only in reducing the applied electric field in the neighborhood of the cathode, but also reduces the effective emitting area of the cathode. Thus it is a common fault that the cross section of the electron beam at its first crossover changes rapidly with modulation and increases with cathode current with consequent non-uniform illumination of the final gun aperture and consequent variation of spot size.

Particularly where high current densities are involved, space charg effects result in a modification in the electrostatic field, causing further aberration. The effects can be allowed for in the design by laborious step by step methods, but these are then usually effective only at one particular value of beam current. The mutual repulsion of the electrons in the beam tends to increase the crossover area and also the divergence of the beam from a crossover, thus reduc-- ing the current efliciency.

It may be said, in general, that with the conventional type of tube using a negatively biassed control grid, while, at the expense of constructional and circuit simplicity, the spot size can be made adequately small and constant, the modulation range is limited and the current efiiciency is low. A measure of the modulation range is the perveance (the ratio of beam current to the three-halves power of the beam voltage at any point) at the modulator electrode; this is usually the order of 0.25 to 0.5 X Current efiiciency is usually subordinated to the other requirements and seldom exceeds about ten per cent; the excess current has to be collected by the accelerating anode and beam stops in the main lens system, which electrodes are usually maintained at high potential.

In accordance with the present invention'there is provided an electron gun for a, cathode ray tube in which an intensity modulated beam of electrons is used for scanning comprising: a thermionic cathode, a first centrally apertured anode and auxiliary electrodes together adapted to form a space charge limited electron beam converging to a crossover whose position and cross-section is substantially independent of positive potential variations applied to thesaid first anode; and a focusing cylinder and a second centrally apertured accelerating anode together adapted to form an electrostatic lens bringing the said beam to a second high potential crossover at the aperture of the said second :anode.

By using the said firstsanode as the modulating electrode a considerable improvement in periormance and simplicity of construction and op- H eration is obtainedin comparison with previously known arrangements. The modulating electrode no longer behaves as the control grid in a triode. Since 'the beam current is dependent merely upon its own potential and not, in part, due to the penetration through the field of a subsequent electrode. This results in the position of the first crossover being independent of the variation of modulating voltage applied to the anode; for if. all the potentials of. a beam forming system are modified by the same proportionate factor, the shape of the beam is unaltered and in the present case there is only one potential involved. Furthermore, not only does the diodeconstruction lead to .a higher perveance at the modulation electrode (in part .due to the more uniform potential gradient at the cathode obtainable with the diode construction), but aberrations in the region of the cathode. can be reduced to very small proportions. By proper design the efiects of variation of space charge can be eliminated so that not only the position but also the size of the crossover may be made independent of the beam modulation. The crossover, however, being formed at the low potential of the modulating electrode is too large to be imaged directly ontothe viewing screen; it is, therefore reformed. at a higher potential. This. higher potential is desirably the maximum available, and in the embodiment to be described is that of the viewing screen.

Strictly speaking it is. the image of the abovementioned second crossover which is to be imagedon the viewing screen. Due to the velocity variation among the electrons: emitted from the cathode there remain some aberrations and this crossover is not sharply defined; a stop must be placed around it tolimit its sizeto anappropriate extent; in practice, therefore, what is finally focussed onto the viewing screen is an image of the aperture of this stop which, for'convenience, is also employed as the said second anode.

In order that the. modulating electrode; the first anode, should collect little; if any current, the design of the; diode section should take this factor into consideration. A suitable electrode. configuration. may be worked. out on electron path-plotting principles, but the fundamentally correct method is the design procedure first is known as the Pierce gun. Since the Pierce gun takes full account of the modification of the electron optical properties of the system due to the passage of space current, it provides a desig giving the maximum modulation perveance consistent with other requirements, a complete utilization of the cathode area, fundamentally no aberrations and little-or no flow of current to the modulating electrode.

As applied to a circular cross section electron beam, the principle of the Pierce gun may be described as follows:

If two concentric spheres be considered, one as a cathode and the other as an anode, an ideal space-chargedimited rectilinear electron flow will take place between them converging to or diverging from the centre of the spheres. If, now,

' all but a conical section of the system be removed,

the apex of the cone coinciding with the centre of the inner sphere, the ideal electron flow in this remaining section can still be maintained by reshaping the existing electrodes and providing additional electrodes of such configurations and potentials thatthey have thesame effect upon the electrons in the section as those parts of the system (including the electronic space charge) which have been removed. Theoretically there is but one solution to the design equations :governing the shape of the electrodes; in practice, however, a sufiioient degree of approximation for any purpose may be obtained by an infinity "of difiering electrode constructions, the precise arrangement of any given type of configuration be ing. obtained by means of electrolytic tank experiments.

An embodiment -of the invention will :be do scribed. with reference to the accompanying drawings, in which:

Fig. 1 shows :a cross-section through: a projee tion type of cathode ray tube suitable for em bodiment of an electron gum according to the present invention, and

Fig. 2 is a diagrammatic sectional view of an electron gun according to the present invention.

In: the drawing of Fig. 1 the cathode ray b31113 comprises the usual neclrxp'ortion' 1 provided with a pressed glass base 2 carrying contact 3 for the low potential electron gun electrodes. The neck portion. connect with the usualb'ul b 4, to the endof which is ius'ed an optically prepared glass .face 5 having a fluorescent screen 6- depos lied on its bacln. The electron beam, issuing from the electron gun indicatedat T is brought to a focus on the screen by means of magnetic iocussirrg arrangements indicated. by the coil 8': horizontal and vertical deflecting coils are indicated by the rectangle 9.

The electron. gun, which is. shown more clearly in 2, comprises-a cylindrical indirectly heated cathode it having a concave emitting surface H- The cathode is supported by means ot-an" insulating washer la in an outer metal cylinder to and is surrounded by a cylindrical electrode M projecting beyond the edge ofthe-cathocle. The cylinder I l has: a flanged base l5- by means of which it is supported between washer ['2 and a further insulated washer rs coaxiall'y with the cathode it and cylinder [3. The first anode is formed: as' a re entrant frusto-eonical portion of. a sheet. metal. member 1 8- Which fits closely inside the cylinder l3 and bears against the washer it. A centrally apertured disc 9 inside: and closes the end or thecylinder I i-F and Iormulatedby J-. R. Pierce resulting in what 15 serves to: shield: the anode-cathode space from I external fields, including the field of the second anode. A cylindrical shield electrode 20 partially closed by a centrally apertured end plate 2| is located in alignment with cylinder I3 by means of an insulating washer 22 and is held in position by means of three insulating bridge members 23 equally spaced around the gun. The second anode 24 forms the base of a cup-shaped member 25 having its wall aligned with the cylinder 20 and its base adjacent the end plate 2|; it is held in position by three ceramic bridge pieces 26 equally spaced around the structure and securing member 25 to cylinder 20. Three spring fingers 21 secured to the second anode member 25 press against the neck portion I when the gun is inserted into the cathode ray tube and contact with a colloidal graphite coating 28 which extends into the bulb 4 and enables contact to be made with a terminal pin 29 and an aluminium backing 30 for the fluorescent screen 6.

After issuing from the aperture 33 in anode 24 the electron beam is focused by conventional means, such as the magnetic focussing arrangement shown in Fig. 1. The arrangement is such as to focus on the screen 6 an image of the aperture 33.

In operation the cylinder I4 is maintained at cathode potential and a modulating potential between 0 and 50 v. positive is applied to the cylinder l3 and to anode IT. The second anode is maintained at 20 kv. The cylinder 20 is normally connected to the cathode Ill.

The electron beam, indicated generally at 3|, is brought to a first crossover 32, invarient in position and size, in the aperture of the disc l9. The electron lens formed by the potentials of electrodes [9, 20, 2| and 24 cause the beam diverging from the crossover 32 to be brought to a second crossover 33 in the aperture of anode 24.

With the above-mentioned voltages, using a cathode of 2.5 mm. diameter, and a second anode aperture of 0.1 mm. diameter, a beam current of 1 ma. may be caused to fall on the screen 4 in a spot 0.1 mm. in diameter, the overall current eificiency being of the order of 70%. It will be seen that the modulation perveance at maximum beam current is approximately 3x10 These figures show a very considerable improvement upon those quoted in connection with earlier types of gun, while the structure remains simple to construct and to operate.

While the principles of the invention have been described above in connection with specific embodiments, and particular modifications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What is claimed is:

1. The electron gun arrangement for a cathode ray tube comprising means for forming a space charge limited beam including a thermionic cathode, an auxiliary electrode spaced from but surrounding said cathode and extending beyond the end thereof and a first anode insulated from said cathode and auxiliary electrode and having an aperture through which the beam passes said aperture being axially spaced from said cathode and auxiliary electrode, means, including said first anode and a shielding electrode bounding with said first anode a confined fieldfree region through which the beam passes, for converging said beam at a cross-over point whose position and cross section are substantially independent of the positive potential variations on said first anode, said shielding electrode having a small opening for passage of the beam and being at the same potential as said first anode, and means for producing a second cross-over of the beam including a second anode.

2. The electron gun arrangement according to claim 1 wherein said first anode is the modulating electrode.

3. The electron gun according to claim 1 wherein said cathode has a concave emitting surface, said first anode comprising a member formed into a frustum of a hollow cone with its apex directed towards the cathode; and an outer hollow metal cylinder surrounding the cathode and anode electrodes, and contacting said anode, said shielding member being a disk, having a centrally located aperture, partially closing said outer cylinder and located in the plane of said first cross-over.

REGINALD ALEXANDER LISTER COLE. ANTHONY VICTOR KRAUSE.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,072,651 Schroeter et a1. Mar. 2, 1937 2,112,378 Nicoll Mar. 29, 1938 2,125,418 Benjamin et al. Aug. 2, 1938 2,152,825 Schlesinger Apr, 4, 1939 2,210,127 Rogowski Aug. 6, 1940 2,227,016 Schlesinger Dec. 31, 1940 2,228,958 Hinsch Jan. 14, 1941 2,268,165 Parker et al. Dec. 30, 1941 2,268,194 Glass Dec. 30, 1941 2,283,041 Broadway May 12, 1942 2,341,764 De Gier Feb. 15, 1944 2,436,265 Pohle Feb. 17, 1948 2,468,136 Taylor Apr. 26, 1949 2,476,060 Moss July 12, 1949 2,540,621 Johnson Feb. 6, 1951 2,555,850 Glyptls June 5, 1951 

